A vulnerable plaque is a kind of plaque on an artery wall characterized by a core layer of dead tissue covered with a very thin fibrous cap. This fibrous cap is prone to rupture, releasing bits of the dead tissue into the bloodstream. This material can then flow to the brain or heart and cause a stroke or a heart attack. Vulnerable plaques are identified in autopsies but existing medical imaging systems do not all provide good enough resolution to faithfully identify the thin structures involved.
Intravascular ultrasound (IVUS) is used to study arterial plaques. Typical IVUS systems use a long, thin catheter with an ultrasonic transducer at the tip. The catheter is inserted into a patient's arteries and rotated to capture a series of lines of image data. One rotational set of those lines can be composed into a 2D display giving a cross-sectional view of the artery. However, since the catheter rotates in an organic environment, subject to non-uniform stresses, the rotational speed is variable. As a result, if the system fires a series of scan lines at regular time intervals, there will not be any intrinsic grouping of scan lines into rotational sets. Instead, systems include a mechanical device that fires each scan line at a specific rotational position.
One such device is a rotary encoder. The hardware inside of a rotary encoder creates on distinct electrical signal per step of rotation. Because the hardware provides steps in powers of two, existing rotary encoders provide, for example, 512 or 1024 steps per revolution. IVUS systems are wired so that each step provides the electrical impulse that fires the transducer. To increase scan line density requires redesigning a system or making a new system to operate with a higher density rotary encoder. However, the problem is compounded by the recognition that rotary encoders can only have so many steps. Too many, and the electronics will not be able to faithfully detect each step speeds typical of IVUS systems.